Many commercial and industrial processes generate large amounts of waste heat which must be removed for successful operation. The waste heat is often carried in the form of a hot fluid stream. For a number of reasons, it is often undesirable or impermissible for the hot fluid stream to be disposed of, so it must be cooled and reused. One such hot fluid stream is exhaust steam, such as from an electric power generating steam turbine, which is condensed to water which then is reconverted to steam in a boiler to be used again in powering the turbine.
A base cooling system of one type or another is provided for cooling the hot exhaust steam. All such systems rely, ultimately, on heat rejection to the environment, either by direct rejection, or indirectly through an intermediate fluid, to air or to water from a river, lake or sea.
A typical cooling system can be illustrated further by reference to a power generating plant. In the production of electric power, heat is first produced by any suitable means such as nuclear energy, geothermal, solar, or combustion of a fossil fuel such as oil, gas or coal. The heat produced is then used to convert water into steam. The steam is conducted at high pressure to a turbine which it drives. The turbine is, of course, coupled to a generator which produces electric power. The spent steam from the turbine is condensed by the cooling system and then the water is recycled and reheated to steam again.
An air-cooled system is generally designed and built to provide a cooling capacity or duty adequate for the intended purpose on the hottest day, or ambient temperature, anticipated at the site of the plant involved. This results in an excess cooling capacity for all but a small number of days out of a year. Even on the hottest days, the maximum cooling capacity of the system often is not utilized except during the very warmest part of the day. This is because the atmospheric temperature from day to night will vary as much as, or more than, 20.degree. to 30.degree. F., making it unnecessary to utilize the maximum cooling capacity of the system most of each day. The cooling system installation, operation and maintenance involve large costs and expenses which cover a system that is not anywhere fully employed, regardless of the hot fluid stream to be cooled.
A water cooled system is generally designed for the highest temperature of the water from the available source e.g. river, lake or sea. The cooling water picks up heat in condensing the steam. The heated cooling water is disposed of into the river, lake or sea.
As an alternative, many power generating plants cool the heated cooling water in an evaporative cooling tower by contacting it with ambient air. Large natural or mechanical draft cooling towers are extensively used for this purpose. Although the heated water is cooled in this manner, a substantial amount is expelled as water vapor which may form artificial clouds leading to fog, ice and other problems, in addition to the loss of increasingly scarce fresh water.
An evaporator cooling tower serving a 1000 megawatt electric generating plant may lose as much as 600,000 gallons of water per hour into the atmosphere. Also, the evaporator towers are susceptible to a large growth of bacteria, causing additional environmental problems.
Various dry-type cooling systems have also been proposed. Some of the these use indirect heat rejection, such as to ammonia, as is disclosed in U.S. Pat. Nos. 4,270,358 and 4,315,404. Other systems employ combinations of direct and indirect systems for condensing exhaust steam as disclosed, for example, in U.S. Pat. Nos. 3,831,667; 3,935,902; and 3,841,100. In the direct cooling aspect of such systems, cooling water is injected directly into contact with the hot exhaust steam after it flows from the turbine under vacuum. The resulting hot cooling water must then be cooled for further use in condensing steam. One way is to pass the hot cooling water through heat exchangers in a cooling tower. Such heat exchangers are expensive and undesirably increase the capital cost of a power generation plant. Furthermore, the cooling tower must be sized in such a system to handle the maximum heat rejection load which may be reached only a few times per year.
While U.S. Pat. Nos. 4,270,358 and 4,315,404 disclose indirect cooling systems for condensing exhaust steam, they also disclose a peak shaving system which can be placed in operation when the heat rejection load exceeds the design capacity of the cooling tower. The peak shaving system employs cooling water, which is stored in a cold reservoir, to help indirectly condense the steam at peak loads. The resulting hot cooling water is then fed to a hot reservoir where it is stored until it can be cooled for reuse. One way to cool the hot cooling water is to pass it through heat exchangers in the cooling tower when there is excess heat rejection capacity available in the cooling tower, such as at nighttime. While such a system is practical, it still involves very high capital investment cost in the cooling tower and ancillary equipment.